Erecting finder



Oct. 9, 1928. 1,687,030

- -G. A. MITCHELL ERECTING FINDER Filed May 22, 1926' (freiem-ea ocr. 9,1928.

UNITED STATES.

PATENT OFFICE.

GEORGE A. IITCHELL, OF LOS ANGELES, CALIFORNIA, ASSIGNOB, TO MITCHELL CAMERA. CORPORATION, A CORPORATION OF CALIFORNIA.

:insomnie FINDER.

Application led May 22,

accurate andreliable finder is desired. as

great accuracy is required in getting exactly the correct field of view on the film. It is also highly desirable that the image in the finding instrument be shown upright and in correct right and left relation, so as to avoid all possible confusion in the mind of the operator. An object of the present invention is to provide a simple form of inding instrument that will show the field of view without inversion or re- V"versa-l 'and will show it of a size suitable forv easy inspection. It is a further particular object to provide an optical structure simple and effective in itself for erecting the image, which would otherwise be inverted and transposed right for left by the ordinary objective lens of the finder.

Other objects and corresponding acconi plishments of the invention will be better understood from a consideration of the following" detailed description, reference for this purpose being had to the accompanying drawings, in which:

Fig. l is a more or less diagrammatic j longitudinal section showing a4 finding in- Y. n the .appended claims.

strument in accordance with my invention;

' Fig. 2 isV a .diagrammatic perspective showing the same;

Fig. 8 isv a diagram illustrating a somewhat modified form of said device g and Fig. 4 is a diagrammatic perspectivel showing another modified form.

It will be understood, and particularly vso from what I say hereinafter, that othe image isreceived. This translucent surface may be any surface suitable for the purpose,

110messala) many suchV being known in the art. Wit-h'- out intending any limitation, I shall here refer to it as a ground glass. Preferably I form this ground glass surface on the front face of a light collecting or slightly magnifying lens 1l. This lens and its front image receiving face may be convenicntly rectangular in shape, its proportions corresponding to the proportions of the picture being received in the camera to which the finding instrument is attached. The ground glass thus forms a frame showing the outlines of the image actually received in the camera.

The grormd glass 10 may be mounted in any suitable tube or other housing 12, which also carries the erecting objective combination 13. This combination may be carriedV in a -separate casing 14, and part 14:a of which may be tubular` in shape so that it may slide in the housing 12 to adjust the distance between the objective combination and the ground glass. It will be understood, however, that to adjust this focal distance it is only necessary to move the ground glass and the object-ive combination relatively, that either one may be moved with reference to the housing. To keep the objective combination always in one relan tive position (upright, as shown in the drawings), a pin 14D may play in a longitudinal Slotv 14 as shown in Fig. l.

The objective combination comprises au objective made up of one or two lenses l5.

For instance, the objective may be formed of two lenses 15 of suitable configuration to obtain an image of the size desired on ground glass l0;` and the two lenses may be aiiixed. as by cementing, to the opposite plane faces of the prism structure of the objective combination, as is shown in the various views. It will also be well understood that a single lens could be used as an objective, but that use of double lenses is usually preferable in an objective for variloo ous well known reasons, and also for the- Y reason that, in-this case, the lightv beam through the prism structure is practically a parallel beam and the prisms therefore need only be of such size as to have reflectjng faces substantially the same size as the enses.

f The function ofthe prisms in the objectivel Y combination is to erect and rectify the-image nally thrown upon the ground glass. l The fonmshown in Figs.' 1, 2 and 3 consists of lll a rilit angled prism 20 to whose hypotenuse face are' cemented two smaller'v right angle prisms 21 and 22 in the arrangement and relation best shown in Fig. 2'.

Supposing the-object to be at 0,'a light ray from the objective traverses the front lens 15,"strikes the 45 face 21a of 'prism 21 and' is reflected downwardly to strike one of the 45 faces of prism 20 and hence to be rc- .flected to the other 45 face of that prism andl then'ce upwardly to strike the 45 face prism 22' and thence rearwardly through the light lens 15 to ground glass 10. The reflection through the. prism structhe magnifying glass. In this case the lens at llperforms the function of a light collector for pile magnifying glass`25, giving good illuml ation 'over the whole image.

In Fig. 4 I show (an objective combination 13a of a somewhat different specific type. Here Ithe two lenses 15 are cemented to the fiat front and rear faces of a prism of special form, this prism having two upper angular reflecting faces and two lower angular reflecting faces` 31. 'This form'of erecting prism is known as the Brashear-Hastings erecting prism. Its, function is substan- :tially vthe same as the. prismatic 'system shown in Fig. 2; and acting in combination with the lens' 15 it obtains the same result as before described. In Fig. 4 the dash-line shows howl a ray wouldproceed through prism structure without lens 15,'indicating how the image, without lens- 15, would be inverted and reversed; and dotted-line 35?* shows how the same ray proceeds throu h lensfor the purpose bf rectifying and erectthe complete combination including tie lenses, indicating that the imagev is erect and in `correct position right for left.

There are other prismatic systems that may be used in connection with an objectiveing the image; but I preferto use a system of such physical proportions that it may be placed between the .two lenses of the objective without separating those lenses too far `to prevent their actingas a singleobjective lens. SIt is, of course, well known thatseveral individual lenses of a complete objective lens may be designed so as to be separated somewhat from each other; and I make use of this possibility in my present design and separate the two lenses. These observations, of course, 'apply'enly to the usev of an objective having twolensjglasses, Ify a .day of April, 1926.

single lens glass is used, that single glass may be cemented to the front face of the prismatic system.

Also, for the purpose ofi-this erecting ob- I jective combination, I prefer to use a pris-v matic system which has fiat .parallel front and rear faces so that the objective lens glasses may be cementedV directly to those faces. venient and compact erectinglehs combination with the prismatic system between the two lens glasses. In such a position, as will readily be understood, Vthe prismatie system need not be very large, as the pencil of light passing between the two lens glasses of theobjective is comparatively concentrated.

By so doing I provide a simple, con- The whole objective combination can fthus i conveniently be made of small size.

Iclaim: 1. In an erecting finder, anerecting ob-4 jective combination comprising an erecting prismatic system having parallel flat front and rear faces, and an objective of two lenses, the lenses being cemented respectively to the front and rear faces of the .prismatic system and an image receiving surfaceffrom the objective combination vand onwhich suri A face the objective throws its image.

2. Inan erecting finder, anerectmg o bjective combination comprising an` erecting prismatic system havingflat parallel front and rear faces each substantially atright angles to the optical axis of the system, and

an objective lens comprising two lens glasses cemented respectively to said'frontand rear faces of the prismatic system, the two lens glasses of the objective acting'v as a single objective lens which would without the interposition of the prismatic erecting system throw'an inverted and reversed image; and

an'image receiving surface spaced from the objective combination and on which surface the objective throws its image. .Y

v3. In an erecting finder, an erecting objective combination comprising an erecting prismatic system, and an objective of two lenses secured directly to the front and rear faces of the prismaticsystem; the lenses beingof'such focal lengths as -to act as a single objective with a focal length that.

throws an image behind the rear lens; 4and an image receiving'- surface behind the rear lens.

4. In an prismatic. system, and an objective 4of two lenses secured directly to the front v and rear -erecting finder, an erecting. ob- I jective combination -comprising an erecting faces vof the `prismatic system; .the lenses being of such focal lengths as to act as a lsingle objective with -a focal length -that throws an imaOe -behindthe rear lens. o

In witness iat I claim thel fregolng-I havehereunto subscribed my. name this 29 GEORGE A. MTGHELL. 

